Burning wet windrows reduces potential for weed seed
destruction
Dr. Michael Walsh1, Vanessa Stewart2 and Peter Newman3
1Western Australian Herbicide Resistance Initiative, School of Plant Biology, University of
WA, Crawley, WA 6009
2Department of Food and Agriculture, Merredin, WA 6415
3Department of Food and Agriculture, Geraldton, WA 6530
KEY MESSAGES

Potential for weed seed destruction during burning remains high for early season burning even after
significant rain fall events

After rainfall in March burning temperatures were reduced but long burning durations maintained a
high potential for weed seed destruction

Much lower temperatures and shorter durations of windrow burning were recorded following rainfall in
April resulting in reduced potential for weed seed destruction
AIMS
This study aimed to determine the effect of large rainfall events (50 mm) in March and April on the burning
temperatures of narrow windrows and subsequent potential for weed seed destruction.
METHODS
Two trials were established in wheat stubble at Merredin in March 2007 by watering windrows in a simulated
rainfall event. The first trial was established in March where simulated rain fall events of 50mm were applied
on three occasions (Table 1) to three replicated 2m2 areas of windrow (i.e. 1m wide by 2m long) by spraying
100 L of water on these sections with a fire hose. These watered windrow sections along with three dry
sections were burnt on the 27th of March. Temperature probes and a data logger were used to measure soil
surface temperatures during burning of each windrow section. This experiment was repeated in April where
windrow sections were treated with 50mm simulated rainfall events as described above. These sections were
subsequently burnt on the 3rd May with burning temperatures recorded as described earlier. In each
experiment soil and stubble samples were collected from each plot prior to burning for the determination of
moisture levels. Collected samples were oven dried for two days at 70C before weighing.
Table 1. Simulated rainfall date and the corresponding number of days prior to burning rainfall
treatments in experiment one and two.
Watering date
Experiment 1
5th March
12th March
19th March
Experiment 2
11th April
18th April
25th April
Days prior to burning
29
22
15
22
15
8
RESULTS
March Burning
Narrow windrow burning temperatures, and therefore, the potential for killing weed seeds were reduced
following a significant rainfall event in both March. In March, the maximum temperatures recorded on the soil
surface during windrow burning were generally lower for all treatments that received 50 mm of rainfall when
compared with the no rain treatment (Figure 1A). Therefore, with lower maximum soil surface temperatures
occurring beneath the windrows there is likely to be a reduced potential for weed seed destruction following a
large rainfall event. However, the durations of high temperatures (> 300C) is still likely to have provided a
high potential for weed seed destruction.
Large rainfall events in March did not result in large increases in low soil and stubble moisture levels and
therefore, delaying burning treatments had no effect on burning temperatures. There were no real differences
in recorded temperatures between windrows that received rain 29 days prior to burning or those that received
rain 15 days prior to burning (Figure 2A). Therefore, although there was likely to be a slightly reduced
potential for weed seed destruction delaying burning did not increase this potential.
March
Temperature (C)
A
No rain
29 days
22 days
15 days
800
700
600
500
400
300
200
100
0
0
125
250
375
500
625
750
875
1000
1125
1250
Time (Secs)
April
B
No rain
22 days
15 days
8 days
Temperature (C)
800
700
600
500
400
300
200
100
0
5
130
255
380
505
630
755
880
1005
1130
1255
Time (Seconds)
Figure 1. Effect of time between 50mm rainfall and burning on the soil surface temperatures during
burning of 23 t/ha wheat stubble in narrow windrows in (a) March and (b) April.
April burning
In April the potential for weed seed destruction was reduced for all treatments with the timing between rain
fall events and burning having a large impact on the potential for weed seed destruction. There were
generally lower burning temperatures for all treatments and particularly following the last two rainfall
treatments (Figure 1B). For these treatments low maximum burning temperatures (< 400 C) were recorded
and the duration of these high temperatures was very short. Therefore, the potential for weed seed
destruction for these treatments was very low.
Higher stubble and soil moisture levels in April were responsible for the lower burning temperatures recorded
for all windrow burning treatments. As well as the simulated rainfall there was approximately 14 mm of rain
fall received across the site during April resulting in much higher stubble and soil moisture levels at the time
of burning (Figure 2B). In particular stubble moisture levels were markedly higher in April than March and
were consequently the primary reason for the generally lower burning temperatures recorded.
A
March
25
Moisture (%)
20
Stubble moisture
Soil moisture
15
10
5
0
No rain
29
22
15
Days since 50 m m rainfall
April
B
25
Moisture (%)
20
15
10
5
0
no rain
22
15
8
Days since 50 m m rainfall
Figure 2. Stubble and soil moisture levels recorded at burning following 50mm rainfall in (a) March
and (b) April
CONCLUSIONS
The potential for weed seed destruction form windrow burning remains high for early season burning despite
the occurrence of significant rainfall events. Although maximum burning temperatures are reduced by rain the
long duration of burning is likely to maintain a high potential for weed seed destruction. However when
burning is delayed until April then rain fall will reduce both maximum burning temperatures and durations
resulting in much lower potentials for weed seed destruction.
KEY WORDS
Narrow windrows, stubble burning, weed seed destruction
ACKNOWLEDGMENTS
The authors wish to thank the WAHRI staff for their contributions and support in the conduct of this research.
This research was financially supported by the GRDC. This research comprises part of Natalie Maguire’s 4 th
year honours project for which she received a Weeds CRC scholarship.
Project No.:
Paper reviewed by:
UWA 399